The present invention relates to improvements in the control of refractive index of films on substrates, and, more particularly, to a method, apparatus and means of fabrication of photosensitive germanium-doped silica films which change their refractive index upon UV irradiation.
Even more particularly the present invention also relates to the fabrication of photosensitive Germanium-doped silica files using plasma enhanced chemical vapour deposition (PECVD).
Photosensitivity in germanosilicate optical-fibres was first observed over 15 years ago. However it was the demonstration UV-written gratings which revived interest in this area. This was followed by writing reflection gratings in the telecommunications window of 1550 nm. These results have stimulated major activity worldwide in this area because such grating devices have a potential major impact on future telecommunications systems.
By coupling UV photosensitivity with planar silica waveguide technology, a very wide range of devices becomes possible and a high degree of integration can be achieved, bringing the benefits of device stability and compactness not available in fibres.
However, prior known germanium doped planar structures have been formed by flame hydrolysis and need to be hydrogen loaded for up to 2 weeks to become reasonably photosensitive. The hydrogen loading introduces unwanted side effects, such as transient behaviour due to gas out diffusion and increased absorption at 1.5 xcexcm. An alternative deposition technique, which is able to produce photosensitive films without hydrogen loading, is Plasma Enhanced Chemical Vapour Deposition (PECVD). A UV-induced positive refractive index change of 0.0025 has been reported for germanium-doped PECVD silica films without utilising hydrogen loading. These xe2x80x9cpositivexe2x80x9d photosensitive Ge-doped films, however, are deposited by standard high pressure ( greater than 50 Pa) PECVD and suffer from a high scattering loss, which makes this material less suitable for the fabrication of planar waveguide devices.
There is therefore a general need for an improved form of silica waveguide which does not suffer, to the same degree, the aforementioned problems.
In accordance with a first aspect of the present invention there is provided a method of fabricating a photosensitive germanium-doped silica film of a wafer, substrate or the like which is adapted to change its refractive index by a predetermined magnitude and sign upon UV irradiation, said method comprising the selection of deposition conditions for plasma enhanced chemical vapour deposition (PECVD) that result in the formation of a non-porous film.
Preferably the PECVD process is operated so as to control the structure of said film and to produce a film which is adopted to increase its refractive index upon UV irradiation. Alternatively, PECVD process is controlled so as to produce a film which is adopted to decrease its refractive index upon UV irradiation.
The PECVD should be carried out with the level of ion bombardment on the film surface during the deposition necessary for the formation of non-porous silica. The value of the change in refractive index can be adjusted by separately or simultaneously varying the level of ion bombardment and/or the temperature of the wafer, substrate or the like or the level of dopants.
GeH4 can be used as source material for germanium doping and preferably the PECVD process is carried out in a high plasma density hollow cathode deposition system comprising two opposing RF powered electrodes which operate to produce a high density plasma between them due to an xe2x80x9celectron mirrorxe2x80x9d effect. The electrode opposing the wafer holding electrode can be partly screed to produce advantageous effects. The wafer holding electrode can be partly screened with a thin solid round plate having a diameter less than the diameter of the electrode and placed parallel to it and separated from it by a distance smaller than half of the inter-electrode distance.
In a second aspect of the present invention there is provided a photosensitive germanium-doped silica film on a wafer, substrate or the like which is adapted to change its refractive index upon UV irradiation, said film formed by plasma enhanced chemical vapour deposition (PECVD) of the film.
Preferably the PECVD process is operated so as to control the structure of said film and to produce either an increase or decrease in refractive meter upon UV irradiation.
The PECVD process is preferably carried out with the level of ion bombardment on the film surface during the deposition necessary for the formation of non-porous silica and the value of the change in refractive index is adjusted by separately or simultaneously varying the level of ion bombardment and/or the temperature of said wafer, substrate or the like or additionally varying the level of germanium doping.
Preferably GeH4 is used as source material for germanium doping.
The PECVD process can be carried out in a high plasma density hollow cathode deposition system comprising two opposing RF powered electrodes which operate to produce a high density plasma between them due to an xe2x80x9celectron mirrorxe2x80x9d effect. One electrode can be partially screened to produce advantageous effects.
The partial screening can be done with a thin solid round plate having a diameter less than the diameter of the electrode and placed parallel to it and separated from it by a distance smaller than half of the inter-electrode distance.
In yet another aspect of the invention there is provided a method of forming an optical signal processing element on a substrate according to the method described above.
In accordance with a further aspect of the present invention there is provided a method of fabricating a photosensitive germanium doped silica film on a substrate the film adapted to change its refractive index upon UV irradiation, the method comprising utilising a plasma enhanced chemical vapour deposition (PECVD) of said film utilising a level of bombardment of the substrate surface sufficient to cause the formation of substantially non-porous silica.
In accordance with a further aspect of the present invention there is provided a method of fabricating a photosensitive germanium doped silica film on a substrate which is adapted to change its refractive index upon UV radiation, said method comprising plasma enhanced chemical vapour deposition (PECVD) of said film utilising a hollow cathode deposition system.
Preferably, the degree of change in refractive index is determined by varying one or more of:
(a) the level of germanium doping
(b) the level of ion bombardment of said substrate
(c) the degree of UV irradiation of said substrate
The invention also has application in changing the refractive index in respect of one birefringent axis with respect to a second birefringent axis.
In accordance with a further aspect of the present invention there is provided an optical waveguide comprising a photosensitive germanium doped silica film on a substrate, said waveguide adapted to change its refractive index upon UV irradiation, said film being formed from substantially non-porous deposition of said film. The deposition can be via plasma enhanced deposition of said film utilising a hollow cathode deposition system and the degree of change in the refractive index is determined by variation of one or more of:
(a) the level of germanium doping
(b) the degree of UV irradiation of said substrate.